Cyclic oligomeric oxo- and imido- metal complexes as olefin polymerization catalysts

ABSTRACT

Single site nitrogen and oxygen metallacyclic catalysts comprising at least two polymerization-stable anionic ancillary ligands and a Group 4-10 metal are disclosed wherein the nitrogen or oxygen heterocyclic catalyst is produced by dimerizing an alkylamido or trimerizing a metal containing oxane. The polymerization-stable anionic ancillary ligands are preferably either cyclopentadienyl, borataaryl, pyrrolyl, azaboralinyl, pyridinyl, or a quinolinyl ring. In a preferred embodiment, the invention relates to catalysts of the formula: ##STR1## wherein M is a transition metal of Groups 3-10; 
     L is a polymerization-stable anionic ancillary ligand; 
     R is a hydrocarbyl group containing up to about 12 carbon atoms optionally substituted with halogen; and 
     E is R or halogen, alkoxy from C 1  to C 20 , siloxy from C 1  to C 20 , N(R) 2 , hydrogen or another univalent anionic ligand, or mixtures thereof.

FIELD OF THE INVENTION

This invention relates to a novel metallocene catalyst system containing a metallacyclic catalyst having at least two polymerization-stable anionic ancillary ligands. The invention further relates to a method of preparation of the catalyst and a method of using the same.

BACKGROUND OF THE INVENTION

Historically, polyolefins have been made with conventional Ziegler catalyst systems. Such catalysts typically consist of transition metal-containing compounds and one or more organometallic compounds. For example, polyethylene has been made using such Ziegler catalysts as titanium trichloride and diethylaluminum chloride, as well as a mixture of titanium tetrachloride, vanadium oxytrichloride, and triethylaluminum.

While these catalysts are inexpensive, they exhibit low activity and therefore must be used at high concentrations. As a result, it is sometimes necessary to remove catalyst residues from the polymer, which adds to production costs. Neutralizing agents and stabilizers must be added to the polymer to overcome the deleterious effects of the catalyst residues. Failure to remove catalyst residues leads to polymers having a yellow or grey color and poor ultraviolet and long term stability. Additionally, for example, chloride-containing residues can cause corrosion in polymer processing equipment.

Furthermore, Ziegler catalysts produce polymers having a broad molecular weight distribution which is undesirable for some applications such as injection molding. They are also poor at incorporating α-olefin co-monomers. Poor co-monomer incorporation makes it difficult to control the polymer density. Large quantities of excess co-monomer may be required to achieve a certain density and many higher α-olefins, such as 1-octene, may be incorporated at only very low levels, if at all.

Although significant improvements in Ziegler catalyst systems have occurred since their initial discovery, they lately have been substantially replaced with "single-site," in particular, metallocene, catalyst systems. A traditional metallocene catalyst typically consists of a transition metal compound which has one or more cyclopentadienyl ring ligands bound in an η⁵ fashion. The cyclopentadienyl ring ligands are polymerization-stable; that is, they remain bound to the metal during the course of the polymerization process. They produce polymers of high molecular weight and display narrow molecular weight distributions, because the cyclopentadienyl ligands deter formation of secondary polymerizing species. These catalysts also incorporate α-olefin co-monomers well. However, at higher temperatures traditional metallocene catalysts tend to produce lower molecular weight polymers. They are particularly useful for gas phase and slurry polymerizations of ethylene, which are conducted at about 80° C. to about 95° C., but are less useful in solution polymerizations of ethylene, at about 150° C. to about 250° C. Additionally, gas phase and slurry polymerizations using supported metallocene catalysts can suffer from sheeting and equipment fouling problems.

Recently, catalysts have been discovered wherein one or more of the cyclopentadienyl ring ligands associated with the traditional metallocene have been replaced by other polymerization-stable anionic ancillary ligands. These may be ligands which are isolobal to cyclopentadienyl; that is, the frontier molecular orbitals--the highest occupied and lowest unoccupied molecular orbitals--of the ligand and those of the cyclopentadienyl ligand are similar. These isolobal ligands may include tris(pyrazolyl)borates, pentadienyl groups, phospholes, and carbollides.

In particular, U.S. Pat. No. 5,554,775, incorporated herein by reference, discloses catalysts wherein one or both cyclopentadienyl moieties are replaced by a boratabenzene moiety including boratanaphthalene and borataphenanthrene. Further, U.S. Pat. No. 5,539,124, incorporated herein by reference, discloses catalysts in which one or both cyclopentadienyl moieties have been replaced by a nitrogen-containing heteroaromatic compound containing a pyrrolyl ring, i.e., an azametallocene, variously substituted. The heteroaromatics disclosed in the latter patent include, e.g., indolyl, isoindolyl, and carbazolyl, and other homologous heteroaromatic moieties. The foregoing heteroaromatic catalysts may be referred to generally as heterometallocenes. In addition, PCT International Application WO 96/34021 discloses azaborolinyl heterometallocenes wherein at least one aromatic ring is complexed with a transition metal. Such rings include both a boron atom and a nitrogen atom. These specifically will be referred to as, e.g., azaborolines and the catalysts derived therefrom as azaborolinyl catalysts. The latter catalysts also fall into the general group referred to as heterometallocenes. The foregoing metallocene and heterometallocene catalysts have been developed to include bulky ligands attached to the aromatic moieties. Increased control of the polymerization process may therefore be provided.

Because supported catalysts are more stable, may produce higher molecular weight polymers, and may produce useful changes in the morphology of the polymer, metallocene catalysts are often used in conjunction with a support, such as silica gel.

For the purposes of the present disclosure, it is to be understood that when the term "metallocene" is used, both traditional metallocenes and heterometallocenes such as those disclosed in the above referenced U.S. patents and applications, including those containing bulky ligands, arc contemplated to fall within the scope of the term. Thus, "metallocene" is considered to be a generic term for all such transition metal-bonded aromatic organic polymerization catalysts. Likewise, it is to be understood that when the term "single-site" catalyst is used, both metallocenes as well as other metal complexes containing polymerization-stable ancillary ligands are contemplated to fall within the scope of the term.

SUMMARY OF THE INVENTION

The invention relates to a novel catalyst system containing a metallacyclic catalyst having at least two polymerization-stable anionic ancillary ligands and a Group 4-10 metal. The catalysts arc produced by dimerizing a complex of the metal and the polymerization stable ligand with an alkyl amine or trimerizing the metal-containing complex with water. The catalyst of the invention exhibits high productivity, good comonomer incorporation, and narrow molecular weight distribution--all of which are desired attributes of single site catalysts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A novel metallocene catalyst for the polymerization of olefin homopolymers and co-polymers is of the general formulae: ##STR2## where M is a transition metal of Groups 4-10, preferably Groups 4-7, more preferably Groups 4-6, and most preferably Groups 4-5, of the Periodic Table;

L is a polymerization-stable anionic ancillary ligand;

R is a hydrocarbyl group containing up to about 20 carbon atoms;

E is R or halogen (preferably--Cl or--Br), alkoxy from C₁ to C₂₀, siloxy (R₁)₃ SiO--, where R₁ is a C₁ to C₂₀ alkyl, dialkylamino (N(R₁)₂), hydrogen or another univalent anionic ligand, or mixtures thereof; preferably E is chloride, methyl, phenyl, benzyl methoxy, ethoxy or siloxy, where R₁ is a C₁ to C₆ alkyl.

The metal, M, may be any Group 4 to 10 metal or a metal from the lanthanide or actinide series. In a preferred embodiment, the catalyst contains a Group 4, 5, or 6 transition metal. In a particularly preferred embodiment, the catalyst contains a Group 4 metal, particularly zirconium, titanium or hafnium.

L may be independently selected from a cyclopentadienyl, boraaryl, pyrrolyl, or azaborolinyl or homologous ligands, or from pyridinyl or quinolinyl radicals.

Typically, L in the catalyst of the invention are the mono-, bi-, or tri- cyclopentadienyl or substituted cyclopentadienyl radicals, especially those of the formulae:

    (C.sub.5 R.sup.1.sub.w).sub.f R.sup.2.sub.s (C.sub.5 R.sup.1.sub.w)(III);

and

    R.sup.2.sub.s (C.sub.5 R.sup.1.sub.w).sub.2                (IV).

wherein,

(C₅ R¹ _(w)) is a cyclopentadienyl or substituted cyclopentadienyl; each R¹ is the same or different and is hydrogen or a hydrocarbyl radical such as alkyl, alkenyl, aryl, alkaryl or aralkyl radical containing from 1 to 20 carbon atoms of which two adjacent carbon atoms may be joined together to form a C₄ -C₆ ring;

R² is a C₁ -C₂₀ alkylene radical, a dialkyl germanium or silicon [such as silyl or a radical of the formula--Si(R₅)₂ wherein each R₅ is H, a C₁ -C₁₀ (preferably a C₁ -C₄) alkyl group, an aryl such as benzyl or phenyl or a benzyl or phenyl group substituted with one or more C₁ -C₄ alkyl groups] or an alkyl phosphine or amine radical bridging two (C₅ R¹ _(w)) rings;

s is 0 or 1;

f is 0, 1 or 2 provided that when f is 0, s is 0;

w is 4 when s is 1; and

w is 5 when s is 0.

Particularly good results are obtained where the cylcopentadienyl ring is of the structure: ##STR3## where each substituent group, R₂, is independently selected from a C₁ to C₂₀ hydrocarbyl group and r is a number from 0 to 5. In the case in which two R₂ groups are adjacent, they can be joined to produce a ring which is fused to the cyclopentadienyl ring. Examples of alkyl substituted cyclopentadienyl rings include n-butylcyclopentadienyl, methylcyclopentadienyl and pentamethylcyclopentadienyl. Examples of fused cyclopentadicnyl ring ligands include indenyl, tetrahydroindenyl, fluorenyl and 2-methylindenyl.

The ligand L for use in the olefin polymerization catalyst for the invention may further contain 4 to 30 carbon atoms and may contain a fused ring, one of which is a pyrrolyl ring. Included within this group are heterocyclic radicals of the formula: ##STR4## wherein, R' is independently hydrogen or R⁸⁰ or with F forms a C₄ to C₁₀ fused ring;

each R⁸⁰ is independently selected from a C₁ to C₂₀, preferably a C₁ to C₆, aliphatic or cycloaliphatic radical; a C₆ -C₂₀, preferably a C₆ -C₁₅, aryl radical, or a C₇ -C₂₀, preferably a C₇ -C₁₅, aralkyl or alkaryl radical;

D independently represents a trivalent atom selected from nitrogen, phosphorus, arsenic, antimony and bismuth; and

F is independently selected from carbon and D.

Exemplary compounds include those wherein R' is--H or a C₁ to C₆ alkyl group or C₆ to C₁₀ aryl group. Preferred compounds include 2-methylpyrrolyl, 3-methylpyrrolyl, 2,5-dimethylpyrrolyl, 2,5-di-tert-butylpyrrolyl, aryl substituted pyrrolyl rings such as 2-phenylpyrrolyl, 2,5-diphenylpyrrolyl, indolyl and alkyl substituted indolyls of the formula:

such as 2-methylindolyl, 2-tert-butylindolyl, 3-n-butylindolyl, 7-methylindolyl, and 4, 7-dimethylindolyl and carbazolyl and alkyl substituted carbazolyls of the formula: ##STR5## where m=0 to 6 for formula VII and 0 to 8 for formula VIII and R' is defined as above. Note that the alkyl and aryl substituents on the pyrrolyl ring-containing ligand are not on the nitrogen atom in the ring but are on the carbon atoms of the ring.

Additional examples of ring structures include:

1-Phospha-2,3,4,5-tetramethylcyclopentadienyl,

1-Phospha-3,4-diphenylcyclopentadienyl,

1-Phospha-3,4-dimethylcyclopcntadicnyl,

1-Phosphaindenyl,

1-Phospha-3-methoxycarbonylcyclopentadienyl,

1,3-Diphospha-4,5-diphenylcyclopentadienyl,

1,2,4-Triphospha-3,5-diphenylcyclopentadienyl,

1,2,3,4-Tetraphospha-5-phcnylcyclopentadienyl,

Pentaphosphacyclopentadienyl,

1-Phospha-3-benzoyloxycyclopentadienyl,

Imidazolyl,

Pyrazolyl,

1,2,3-triazolyl,

1,2,4-triazolyl,

Tetrazolyl, and

Pentazolyl.

Still further, the ligand L may be of the formula: ##STR6## wherein R₁₀ is R₂₅, alkaryl from C₆ to C₁₂, aralkyl from C₆ to C₁₂, hydrogen, or Si(R₂₅)₃, R₂₅ is alkyl from C₁ to C₁₂ or aryl from C₆ to C₁₂, R₂₀ is R₁₀ or COR₂₅, R₃₀ is R₂₀, OR₂₅, N(R₂₅)₂, SR₂₅ ,or a fused ring system and n is 0 to 3.

The R₂₅ group is preferably alkyl from C₁ to C₄, the R₁₀ group is preferably C₁ to C₆ alkyl or --Si(R₂₅)₃ and the R₃₀ group is preferably hydrogen or methyl. Examples of fused ring structures that can be used include: ##STR7##

The L ligand may further be a boratabenzene ligand. A boratabenzene ring has the structure: ##STR8## where R₄₀ can be hydrogen, N(R₅₀)₂, OR₅₀, or R₅₀, where each R₅₀ is independently selected from alkyl from C₁ to C₁₀, aryl from C₆ to C₁₅, alkaryl from C₇ to C₁₅, and aralkyl from C₇ to C₁₅. The R₄₀ group is preferably -N(R₅₀)₂ or phenyl and, if R₄₀ is -N(R₅₀)₂, then the R₅₀ in -N(R₅₀)₂ is preferably methyl.

Exemplary of the boratabenzene ligands include: ##STR9## where "p" is 0 to the maximum number of substitutable positions, and is preferably 0. Each R₆₀ is independently selected from halogen, alkoxy from C₁ to C₁₀, and R₅₀. Particularly preferred boraaryl ligands are 1-methyl boratabenzene, 2-phenyl-2 boratanaphthalene, 9-mesityl-9 borataanthracene, and 1-methyl-2-trimethylsilyl boratabenzene

Still, L may be selected from pyridinyl or quinolinyl radicals of the formula: ##STR10## wherein R¹¹ is independently selected from R₈₅, C₁ to C₆ alkoxy, C₇ to C₂₀ alkaryl, C₇ to C₂₀ aralkyl, halogen, or CF₃ ; each R₈₅ is independently selected from hydrogen, C₁ to C₆ alkyl, or C₆ to C₁₄ aryl and Y is O, S, NR₈₅, PR₅, ##STR11## wherein R⁶⁵ is a C₁ to C₆ alkyl and y is 1 to 4.

The single site oxo catalysts of the invention can be prepared by trimerizing the alkylated metal derivative of the polymerization stable ancillary ligand with water. To obtain the single site oxo catalysts in the halogenated form, one can treat the halogenated metal derivative of the polymerization stable ligand with water in the presence of pyridine or a trialkylamine. The single site imido catalyst of the invention may be prepared by dimerizing an alkylated metal derivative of the polymerization stable ancillary ligand by reaction with an alkylamine. Analogous to the oxo catalyst, the halogenated form of the imido catalyst can be produced using the halogenated form of the metal derivative of the polymerization stable ligand. It is preferred that a trialkyl amine be present when preparing the halogenated form.

The reaction is preferably performed by dissolving the reactants in an organic solvent which does not have an active proton such as tetrahydrofuran, anisole, or ethyl ether. The solution should be as concentrated as possible to reduce the amount of solvent that must be handled.

Stoichiometric quantities of water or amine are typically used. The by-products arc removed by filtration, the solvent is evaporated, and the metal ligand catalyst is collected.

The catalysts of the invention produce resins that have a narrow molecular weight distribution and typically exhibit a ratio of MI₂₀ vs. MI₂ between about 10 to about 25.

Since the catalyst of the invention is normally used in conjunction with a co-catalyst, it is preferable to dissolve the organometallic compound in a solvent in which the co-catalyst is also soluble. For example, if methylalumoxane (MAO) is the co-catalyst, then toluene, xylene, benzene, or ethyl benzene could be used as the solvent.

Representative co-catalysts for use in the invention include alumoxanes optionally with aluminum alkyls of the formula Al(R⁷)₃ where R⁷ independently denotes a C₁ -C₈ alkyl group, hydrogen or halogen. Exemplary of the latter of such co-catalysts are triethylaluminum, trimethylaluminum and triisobutylaluminum. The alumoxanes are polymeric aluminum compounds typically represented by the cyclic formulae (R⁸ --Al--O)_(s) and the linear formula R⁸ (R⁸ --Al--O)sAlR⁸ wherein R⁸ is a C₁ -C₅ alkyl group such as methyl, ethyl, propyl, butyl and pentyl and s is an integer from 1 to about 20. Preferably, R⁸ is methyl and h is about 4 to about 10. Representative but non-exhaustive examples of alumoxane co-catalysts are (poly)mcthylalumoxanc (MAO), ethylalumoxane and diisobutylalumoxane. Examples of suitable co-catalysts include MAO and mixtures of MAO with other aluminum alkyls such as triethylaluminum, trimethylaluminum, tri-isobutylaluminum, ethylalumoxane, or diusobutyl alumoxane. The preferred co-catalyst is MAO as it results in high catalyst activity, good comonomer incorporation, and a polymer having a narrower molecular weight distribution.

The co-catalyst can further be a substituted or unsubstituted trialkyl or triaryl boron derivative, such as tris(perfluorophenyl)boron as well as ionic compounds such as tri (n-butyl)ammonium tetrakis(pentafluorophenyl)boron or trityl tetrakis(perfluorophenyl)boron which ionize the neutral metallocene compound. Such ionizing compounds may contain either an active proton, or a cation which is associated with, but not coordinated or only loosely coordinated to, the remaining ion of the ionizing compound. See, for instance, U.S. Pat. Nos. 5,153,157; 5,198,401; and 5,241,025, all of which are herein incorporated by reference. It is preferable not to premix the catalyst and the co-catalyst as this may result in lower catalyst activity. Rather, the catalyst and co-catalyst are preferably injected separately into a reactor containing the monomer to be polymerized. And, preferably, the co-catalyst is injected first. The amount of cocatalyst used with the transition metal compound can be in a molar ratio ranging from about 1:1 to about 15,000:1.

The catalyst and co-catalyst can also be used on a support such as silica gel, alumina, magnesia, or titania. Supports are not generally preferred as they leave additional contaminants in the polymer. However, a support may be required depending upon the process being utilized. For example, a support is generally needed in gas phase polymerization processes and slurry polymerization processes in order to control the particle size of the polymer being produced and in order to prevent fouling of the reactor walls. In order to use a support, the catalyst is dissolved in a solvent and is deposited onto the support material by evaporating the solvent. The cocatalyst can also be deposited on the support with the catalyst or it can be introduced into the reactor separately from the supported catalyst.

Once the catalyst has been prepared it should be used as promptly as possible as it may lose some activity during storage. Storage of the catalyst should be at a low temperature, such as -100° C. to 20° C. The catalyst is used in a conventional manner in the polymerization of unsaturated olefinic monomers, particularly ethylene and propylene.

The catalyst is also useful for copolymerizing mixtures of ethylene with unsaturated monomers such as propylene, 1-butene, 1-hexene, 1-octenc, and the like; mixtures of ethylene and di-olefins such as 1,3-butadiene, 1,4-hexadiene, 1,5-hexadicne, and the like; and mixtures of ethylene and unsaturated comonomers such as norbornadiene, ethylidene norbornene, vinyl norbornene, and the like.

While unsaturated monomers such as styrene can be polymerized using the catalysts of this invention, it is particularly useful for polymerizing α-olefins such as propylene, 1-butenc, 1-hexene, 1-octene, and especially ethylene.

The catalysts of this invention can be utilized in a variety of different polymerization processes. They can be utilized in a liquid phase polymerization process (slurry, solution, suspension, bulk phase, or a combination of these), in a high pressure fluid phase, or in a gas phase polymerization process. The processes can be used in series or as individual single processes. The pressure in the polymerization reaction zones can range from about 15 psia to about 50,000 psia and the temperature can range from about -100° C. to about 300° C.

EXAMPLES Example 1

To a solution of trimethyl(pentamethylcyclopentadienyl)titanium (0.33 g, 1.45 mmol) in 20 ml THF was added a solution of pyridine (0.1085 g, 1.37 mmol) in 4 ml THF at room temperature, followed by slow addition of water (0.026 g, 1.44 mmol) in 4 ml THF. After being stirred for 0.5 h and then filtered, the solvents were evaporated under vacuum to give a yellow solid (0.25 g) whose ¹ H NMR spectrum in CDCl₃ showed it to be the desired product.

This example describes the synthesis of the catalyst having the structural formula:

Cyclic(pentamethylcyclopentadienyl)(methyl)-oxotitanium trimer ##STR12## where Cp* is pentamethylcyclopentadienyl.

Example 2

A cooled (-50° C.) solution of (pentamethylcyclopentadienyl)titanium trichioride (0.4 g, 1.38 mmol) in 7 ml dry dichloromethane was treated dropwise with 0.44 ml (4.15 mmol, 3 equivalents) of tert-butyl amine. The bath was allowed to warm up to room temperature. During the warm-up, the solution color changed from deep red to a lighter red. The mixture was then stirred overnight with the reaction flask wrapped with aluminum foil to protect from light. The solvents were evaporated under vacuum and the residue was treated with 20 ml hcxanes and 5 ml toluene, then filtered. The resulting orange filtrate was evaporated to dryness to afford 0.25g of the desired product.

This example describes the synthesis of cyclic (N-tert-butylimido) (pentamethylcyclopcntadienyl) titanium chloride dimer: ##STR13##

Examples 3-6

These examples demonstrate olefin polymerization conditions and polymer characterizations prepared in the presence of the catalyst of the invention.

The catalyst prepared in Example 1 was used to polymerize ethylene to polyethylene in Examples 3, 4, and 5 while the catalyst prepared in Example 2 was used to make polyethylene in Example 6. The polymerizations were conducted in a stirred 1.7 liter stainless steel autoclave at 80° C. and 110° C. Dry oxygen-frce toluene (840 ml) was charged to the dry oxygen-free reactor. 10% MAO in toluene (from Ethyl Corporation) was typically added with syringe without further purification. A solution of catalyst was prepared by dissolving 0.100 grams of product in 100 ml of toluene. The reactor was then heated to the desired temperature and sufficient ethylene was added to bring the reactor pressure to 150 psig. The reactor was allowed to equilibrate at the desired temperature and pressure. The desired amount of catalyst solution was then added.

At the end of one hour the ethylene flow was stopped and the reactor was rapidly cooled to room temperature. The polymer was filtered from the toluene, dried in a vacuum oven, and weighed. In the following, Exhibit 1 lists polymerization conditions and Exhibit 2 the results of polymerizations.

The melt index of the polymer was measured according to ASTM D-1238, Condition E and Condition F. MI is the melt index measured with a 2.16 Kg weight (Condition E). HLMI is the melt index measured with a 21.6 kg weight (Condition F). The melt flow ratio (MFR) is defined as the ratio of HLMI (or MI₂₀) and MI (or MI₂) and is a measure of molecular weight distribution. A MFR below 25 indicates narrow molecular weight distribution and is likely to demonstrate improved properties characteristics of a single site catalyst or metallocene. Typically a Ziegler-Natta catalyst yields polymer with a MFR of around 30 or greater.

Exhibit 1. Polymerization Conditions

    __________________________________________________________________________          Reactor hydrogen                                                             Temp., Time, delta P,  Catalyst, Co- Al/M                                     Example ° C. min psi Comonomer mmoles catalyst (molar)                __________________________________________________________________________     3    80   60  0    None  0.0067                                                                               MAO 1343                                          4 110  60  0 None 0.0027 MAO 3358                                              5 80 60 10 None 0.0047 MAO 2141                                                6 80 60 10 None 0.0052 MAO 1923                                              __________________________________________________________________________

Exhibit 2. Polymerization Results

    ______________________________________                                                           Catalyst                                                         Activity MI.sub.2 MI.sub.20                                                  Example Wt. PE g kg/g M/hr dg/min dg/min MFR                                 ______________________________________                                         3      4.2        13,100   --      --    --                                      4 4.3 33,000 -- -- --                                                          5 7.0 31,000 0.30 3.75 12.7                                                    6 9.3 55,800 0.20 1.74  8.6                                                  ______________________________________                                     

What is claimed is:
 1. A catalyst system which comprises:(a) a co-catalyst; and (b) a metallacyclic catalyst of the formula: ##STR14## wherein: M is a Group 4 to 10 transition metal;L is a polymerization-stable, anionic ligand selected from the group consisting of substituted or unsubstituted cyclopentadienyl, boraaryl, pyrrolyl, azaborolinyl, pyridinyl, and quinolinyl; R is C₁ -C₁₂ hydrocarbyl or halogen-substituted hydrocarbyl; and E is selected from the group consisting of R, NR₂, hydride, halide, C₁ -C₂₀ alkoxy, and C₁ -C₂₀ siloxy.
 2. The catalyst system of claim 1 wherein M is a Group 4 to 6 transition metal.
 3. The catalyst system of claim 1 wherein M is a Group 4 transition metal.
 4. The catalyst system of claim 1 wherein the co-catalyst is an alumoxane.
 5. The catalyst system of claim 1 wherein the co-catalyst is a triaryl or trialkylboron compound or an ionic borate.
 6. A supported catalyst system of claim
 1. 7. A method which comprises polymerizing ethylene or a mixture of ethylene and an α-olefin in the presence of the catalyst system of claim
 1. 